The selection of ELISA cut-off points for testing antibody to Newcastle disease by two-graph receiver operating characteristic (TG-ROC) analysis.

Two hundred and sixty field serum samples were tested for Newcastle disease (ND) antibodies using a commercial enzyme-linked immunosorbent assay (ELISA) and the haemagglutination inhibition test (HI). The HI test was regarded as the reference method. Reciprocal titres of 16 and above were considered positive. In this study the two-graph receiver operating characteristic (TG-ROC) analysis was used as a tool for selecting cut-off points. Sensitivity, specificity, efficiency and Youden's index were used as indices of test accuracy. The positive and negative predictive values of the ELISA results were analysed for various prevalence rates.